Light is defined as electromagnetic radiation within the wavelength range that stimulates the human visual system, namely 380 to 780 nanometers (see figure below). Immediately adjacent to light in the electromagnetic spectrum are ultraviolet radiation and infrared radiation. Many light sources emit radiation in all three of these wavelength regions. It is worth noting that other animals use slightly different ranges of wavelengths than humans. For instance, snakes and mosquitoes both have rudimentary vision in the infrared, while many fish, birds, reptiles, amphibians and insects can see in ultraviolet.

For the purposes of lighting, light can be considered to travel in straight lines although its direction can be altered by the processes of reflection, refraction and scattering. Depending on the amount of light available and the wavelengths present, people may perceive different colors. These colors can be changed by wavelength-selective filtering or reflection. Light can also be polarized to various degrees which will change its reflection and transmission properties (For more details on the physics of light, see the IESNA Lighting Handbook).

There are many different sources of light. By far the most important natural source of light is the sun. Light from the sun can be seen directly on a clear day, after scattering in the atmosphere on a cloudy day and after reflection from the moon at night. Although the sun emits electromagnetic radiation over a very broad range of wavelengths the peak of its emission is concentrated in the range of wavelengths that stimulate the human visual system (see figure below).

Solar SpectrumLight Figure 2. Wavelengths emitted by the sun and incident on the surface of the Earth. + Click image to enlarge.

As for artificial light sources, these can take many forms ranging from chemical through electrical to biological. Today, the most common artificial light sources use electricity as the power for their light generation. Even so, they can vary widely. The earliest electric lamps generated light by means of a heated filament. These were followed by discharge light sources and more recently by solid state light sources. The result has been a plethora of light sources, each with different characteristics such as light spectrum, luminous efficacy, life, size and cost. The Sources & Auxiliaries section reviews these lamp types in more depth.